Inductor type furnace



April 5, 1932. E. F. NORTHRUP I 3 9 INDUCTOR TYPE FURNACE Filed Oct. 16, 1928 2 Sheets-Sheet l April 5, 1932. E. F. NORTHRUP 135292115 INDUCTOR TYPE FURNACE Filed Oct. 16, 1928 2 Sheets-Sheet 2 g LF Patented Apr. 5, 1932 UNITED STATES PATENT OFFICE EDWIN FITCH NORTHRUP, OF PRINCETON, NEW JERSEY, ASSIGNOR TO AJAX ELECTRO- THERMIC CORPORATION, OF AJAX PARK, NEW JERSEY, CORPORATION OF NEW JERSEY IND'UCTOR TYPE FURNACE Application filed October 16, 1928. Serial No. 31237 My invention relates to combined heating and stirring connections for electric furnaces whereby both the heating and the stirring may be performed to the best advantage.

ne purpose of my invention is to heat and to stir the furnace by electric currents of very considerably different frequency.

A further purpose is to pass higher frequency heating current and lower frequency stirring current through the same furnace inductor coils, preferably at the same time, to perform thereby the preferably coincident double function of heating and stirring to the best advantage.

A further purpose isto. connect higher and lower frequency currents to the same inductor coils, using frequency traps where needed to prevent the current of either of the frequencies from shunting through the connection of the other.

A further purpose is to connect higher and lower frequency coils to the same inductor coils, connecting one to flow through the coils in parallel and the other to flow through the coils in series, and using frequency traps to prevent one current from short-circuiting through the connections of the other current.

A further purpose is to supply higher and lower frequency to the same inductor coils applying the current of one of the frequencies by connections to the coils in parallel from one set of coil terminals and the other through the coils in series and preventing leakage from the series connections through the parallel connections.

A further purpose is to apply low or medium frequency stirring to an existing furnace inductor coil utilizing a higher frequency.

A further purpose is to apply currents of higher and lower frequencies to a furnace inductor for purposes of heating and stirring molten metal in the furnace, and to separate the currents to cause them to pass through their respective paths by appropriate condensers or inductance to block flow of current of one frequency through the connections for application of current of the other frequency.

A further purpose is to connect higher and lower frequency currents to the turns of two connected furnace inductors comparable to adjoining arms of a \Vheatstone bridge, connecting the current of one frequency across diagonally opposite bridge terminals and the current of the other frequency across the intervening diagonally opposite terminals and using frequency traps to form the two intervening connected adjoining arms of the bridge.

A further purpose is to utilize reversely wound furnace inductors to secure induction in the same direction in the melt from high frequency current passing through the inductors in parallel and medium or low frequency in series through the inductors to induce current in the melt in either the same or opposite directions from the two inductors to heat with the former and to stir with the latter.

A. further purpose is to utilize relatively low frequency current through the length of a furnace inductor for stirring purposes whether the current pass through the entire coil in the same direction or reverse its direction.

A further purpose is to use the tuning of a divided circuit to the frequency of a high frequency furnace supply as a trap to prevent the passage of high frequency current therethrough, while using part of the circuit to pass a low or medium frequency current.

A further purpose is to apply a separate part of an inductor coil length to furnacestirring purposes at a different frequency from the frequency of the heating current of the furnace.

Further purposes will appear in the specification and in the claims.

My invention relates both to the methods or processes involved and to circuits and mechanism by which the methods or processes may be carried out.

I prefer to illustrate my invention by diagrammatic views only and to illustrate but a few of the many forms in which my invention may be carried out, selecting forms that are practical, efficient, simple and reliable,

but which have been selected for the purpose,

' diagrammatic, and, except for Figure 1 which usually with a view both to economical gen-.

erator manufacture and use and economical condenser power factor correction.

The heating effect of the current varies with the frequency and also with the square of the ampere turns. On the other hand the stirring effect of the current varies with the square of the ampere turns and not with the frequency.

At'higher frequencies it is possible to obtain the desired heating effect with relatively low current values but with correspondingly lowered stirring effect. It is my purpose to secure highly advantageous stirring also without interfering with the heating effect, nor with the selection of any frequency which may be desired for it wholly independently of the desired stirring. I secure this result by applying two different frequencies which may be applied concurrently or successively. It is much more advantageous to apply the two frequencies together than successively, in order not to lose the time in heating or stirring while current of one frequency is withdrawn in order that the other frequency may be used to stir or heat.

I believe that I am the first to apply current of different frequencies to an inductor furnace for the purpose of heating with a higher frequency and stirring with a lower or commercial frequency and that I am the first also to apply these frequencies coincidently, to apply them to an inductor either wholly or partly .surrounding a molten charge, or to apply them either through the same inductor coil turns or through different inductor coil turns. I believe that I am also the first to apply two different frequencies to the same inductor coils while protecting against the current of one frequency passing through the connections bywhich the other frequency is applied, the first to use frequency traps for such protection, the first to apply inductance for such a purpose so wound that it does not interfere with passage of high frequency current but greatly reduces or blocks passage of low frequency current, and the first to apply condensers to prevention of passage of the lower of two frequencies so used through the connections for applying the other frequency.

In my drawings, I have shown the two frequencies of supply applied in a variety of ways. \Vhere applied concurrently they are shown as passing through the same coils in series for both frequencies, or through the coils in series for one and in parallel for the other, using frequency traps to prevent intermingling of the currents of the two frequencies except as they pass through the inductor coils and many of them balancing the coils as in a Wheatstone bridge. The two frequencies are also shown using independent coils in which they may be applied either concurrently or successively, and as passing through the same coils alternately as distinguished from keeping both heating and stirring currents on concurrently.

As it is more convenient to describe the invention beginning with those forms in which the windings and connections are balanced, which may not inaptly be compared with the balanced arms of a Wheatsone bridge I will begin with this form.

As is well known, the Wheatstone bridge symbolized in Figure 1 as having arms a, b, 0 and 03 may have the source of current 15 in the middle connecting with terminals 16 and 17 and the current indicator 18 in the place where the applied current is generally located, connecting with terminals 19 and 20. This is a mere rearrangement of the dia gram.

If the source of current can thus be placed in either position without disturbing the indicator, two sources of current can be used in the two p0sitions 15 and 18 without the current from either source passing through the other. i

In Figure 2 the bridge arrangement has been applied to a furnace construction, and sources of current 15' and 18 of markedly different frequency have been located at the two positions. The arms I) and d are here two oppositely wound inductors 21 and 22 making up together a total furnace winding adapted to surround the melt. They are separately compensated for power factor by bridged condenser capacity 23 and 24. The arms a and 0 may contain either ca acity, inductance or resistance, of which I have illustrated capacity at 25 and 26. The use of resistance is wasteful of energy but is included in order that the discussion may be complete.

From Figure 1 it will be evident that if the arms be properly balanced current from the higher frequency source 18' will notbe transmitted to or through the lower frequency source 15, and likewise current from the lower frequency source 15 will not be transmitted to nor effect the higher frequency source 18. The lower frequency may advantageously be a commercial frequency such as 60 cycles.

The higher frequency current from source 18 passesthrough the arms a and c and.

through the inductors 21 ancl22 in parallel.

Since the inductors are oppositely wound the heating currents induced in the melt by the current in the two inductors will be in the same direction.

In the absence of a frequency trap or other means to prevent it-the lower frequency current would divide through the inductors in series and through the arms a and 0 in series. It is prevented from doing so in the illustration by the condensers and 26 which have been designed for high frequency and which act as frequency traps for the lower frequency. The lower frequency, therefore, is held to its path through the inductors 21 and 22 in series.

For the stirring purpose, the fact that the current passes through the turns of the two reversely wound inductors in opposite directions, so as to induce current in the melt also in opposite directions, makes little difference. Motor effect circulation is not dependent upon the absolute direction of flow of current but depends upon the fact, true in regard to both of the inductors 21 and 22,.that the (secondary) current induced in the bath by each inductor will be opposite in direction to that of theprima'ry inducing the current.

However, ifit be desired to avoid a slight overlapping of the fluxes of the low frequency field of the two inductors in order to get a maximum stirring effect from them,

with a minimum of intermingling I of the fields due to the inductors, the coils may be spaced a short distance longitudinally of the furnace. The spacing may desirably approximate twice the distance from the molten metal to the coil.

In the form of Figure 2 the capacities in arms a and 0 could be replaced by inductances as shown in other figures. Resistances could be used here and would serve to cut down the flow of the current of lower frequency in arms a and c and hence to increase the flow of this current through the inductors but are very much less desirable than capacities and inductances because of the PR losses (if the currents of both frequencies which pass through the resistances. They are included to complete the discussion.

The same arrangement of parts is shown in Figure 3 as in Figure 2, with three exceptions. Inductances 27 and 28 in branches a and c afford the frequency traps, as distinguished from the traps shown by condensers 25 and 26 in Figure 2; the current supply of higher frequency as shown is adapted for connection to the inductors through taps 29, 29, so as to vary the number of turns of the inductors included in the parallel connection; and the condensers 23, 24' are also shown as adjustable.

The use of inductanccs instead of capacities is of further advantage in this form because of the relation between the inductances shown, in that they are oppositely wound upon the same magnetic core 30 (Figure 12) to neutralize each other with passage of high frequency current through both of the branches at and 0 in parallel, but to add their inductances to prevent short-circuiting leakage of lower frequency current through them which should pass through the furnace inductors. I

The tap connections provided in Figure 3, by which larger or lesser parts of the inductors'may be included in the parallel connection, provide variation of the voltage upon the condensers 23, 24' with consequent variation in the current passing through the inductors. It is quite desirable to tune, or approximately tune, the inductor circuits with current passing through the tuned circuit containing induction and condenser power factor correction, will have many times the value of the current passing through the higher frequency generator.

' It is evident that any of the other higher frequency connections shown may be made to the inductors through taps corresponding generally with those in Figure 3 where it desirable to vary the number of inductor turns bridged by the connection.

Adjustability of the con denser power factor correction is shown for the purpose, chiefly, of indicating that the power factor correction can be accommodated to any conditions of the melt or types of melt, or to any adjustments of the number of inductor turns bridged by the higher frequency supply.

Both of these-forms of Figures 2 and 3 as well as forms appearing in Figures 5, 6, 7 and 8 have connections of what I have referred to as the bridge form, and use two inductors which may be wound in the same direction or in opposite directions as seen.

All but the form of Figure 7 are particularly suited to concurrent application of the higher and lower frequencies of current supply. As the two circuits operate independently, in the sense that neither one disturbs the operation of the other, either supply can be cut off at will when the application of the other one alone is desired, and the form of Figure 7 is capable of concurrent application of the two frequencies when desired.

It is necessary of course, to limit the sum of the two currents passed through any inductor to that current which the inductor will carry.

I have referred to the lower frequency and the higher frequency respectively to point the fact that what is called lower frequency need not necessarily be commercial frequency, and what is called higher frequency may not come within the measure of what by some would be called high frequency. There must, of course, be a decided difference in the frequencies or there is no advantage in apthe result that the higher frequency plying both, since the second frequency would not improve the heating or stirring, as the case may be, sufliciently otherwise to justify its use. My invention will of course find its greatest utility in places where the lower frequency current supplied is taken from commercial alternating current feed lines, and in which the heating current will be of a frequency very much higher than this.

The advantage of applying the heating current and stirring current concurrently lies in the saving of time which would be required to apply them successively, and avoidance of any question of undue stirring o1 heating While the heating or stirring is effected. At the same time, my construction makes it pos sible to operate from one source of supply alone, permitting application of heating current alone or concurrent supply of heating and stirring currents up to the point where a stirring operation is required, with subsequent stirring alone, or heating alone, as the case may be, during the treating.

In Figure 4 a form is shown which is important because it is a. good form with a view to construction and is also well adapted for use in conjunction with a high frequency melting furnace to which it is desirable to apply an intermediate or low frequency stirring' current.

In this Figure 4 a single inductor 21 only is shown, adapted to be supplied with taps 29, 29, if desired, and to which inductor the higher frequency is connected through condensers 25 and 26. Power factor correction is provided by condenser capacity at 23 Lower frequency current is provided through any suitable source 31,. preferably of commercial frequency-say, 60 cycles--connected through transformer 32, so as to apply the current to the terminals 33 and 34 of the inductor.

A circuit 35 is .shown, comprising a coil 36 high in inductance and very low in resistance, and a capacity 37. The circuit is tuned to the higher frequency and constitutes a frequency trap to prevent leakage of higher frequency current through it though the lower frequency passes through the inductance coil 36. The condensers 25 and 26 form frequency traps for the lower frequency current. Neither current can thus short-circuit through the connections for the other.

I have inserted Figure 5 for the purpose of showing that even where. it is desired to utilize the general connections, of Figures 2 and 3, for example, in which the two inductors are oppositely wound, it is not necessary that these inductors be in factwound' oppositelv. The connections of Figure 5 are effectivelv those of Figure 4, except as to the connection by Which the high frequency is run through the inductors 21 and 22 in parallel, so as to produce heating flux in the same direction within the furnace pool, and the stirring current is passed through the two inductors in series with the effect of producing fluxiin the pool in opposite directions. The connections for the higher frequency supply are made at 16 and 17 at one end and 20, 20 (the intermediate connection common to both inductors) at the other, and the lower frequency supply is connected across terminals 16, 17, so that the parallel flow of h gher frequency (heating) current passes through the inductors from the top downwardly in the same direction, giving uniform direction of flux throughout the furnace. On the other hand, the lower frequency stirring connections are made with the upper end of the upper inductor, through the length of this inductor and from it to the lower end of the lower inductor, upwardly through this coil and return ngto the generator, giving reverse direction of magnetic flux through the two inductors.

The form shown in Figure 6 corresponds generally with the forms shown in Figure 5, except that the connections of the higher and lower frequency current supplies to the inductors are relatively reversed so that the higher frequency current passes through both inductors in series, from terminal 16 to term nal 17. and the lower frequency supply is connected to pass through the two inductors in parallel by connecting one lead from the lower frequency supply to the terminal 20 between the inductors and the other lead from the lower frequency supply through counterbalancing induction coils 27 and 28 in parallel to the inductors 21, 2,2 and through them in parallel.

Here the winding of the coils 27, 28' should he in opposite directions as before, in order that they may be non-inductive to passage of current of lower frequency through them in parallel and highly inductive for passage of current of higher frequency through them in series.

Where for any reason it is desirable to apply currents of the different frequencies to the furnace one at a time rather than concurrently, a construction can he used corresponding generally with any of the other figures, but with means for interruptingor check ng the flow of current of one of the frequencies while current of the other frequency is being applied, and vice versa.

In Figure 7 for completeness of illustration I have shown a form in which high resistances 38 and 39 are used to prevent shortcircuiting of a low frequency current supply applied to the terminals 16 and 17 of the reversely Wound inductors 21 and 22. The current is supplied through a transformer 32 from any suitable lower frequency source 31 and short-circuiting of the lower frequency current through the branches a and c is prevented by the resistances when the lower frequency is applied by closing switch 40.

When it is desired to apply the high frequency supply through the arms a and 0 to the inductors in parallel as in Figure 3, for example, the resistances 38 and 39 are cut out by closing switches 41 and 42 in shortcircuits 43 and 44. Switches 41 and 42 must not be closed, of course, while switch is closed, as the lower frequency current would then be short-circuited through the branches at and 0.

Where in such a construction as in Figures 3, 5 or 6 it-is desirable to use independent inductances which do not compensate each other in parallel use as do those of Figure 12, for example, these inductances may be placed in the same general relation as the resistances in Figure 7, and may be used in the same general way, namely, by short-circuiting them when the higher frequency heating current is to be applied and using them in the circuit when the lower frequency stirring current is used.

In both of these forms of Figures 7 and 8 the switches may be interlinked to operate in any desired relation, such, for example,

that switches 41 and 42 shall remain open when switch 40 is closed and vice versa.

In the form shown in Figure 9 both higher frequency and lower frequency currents are passed through the inductors 21 and 22 in parallel from any suitable higher frequency source 18 and the secondary of the transformer 32 fed by a source of lower frequency 31, respectively. A higher frequency capacity 45 in series with the higher frequency supply and an inductance 4b in series with the transformer secondary act as wave traps for the lower frequency and higher frequency currents respectively. In this form it: is desirable to have the turnsof the two inductors oppositely wound, one with respect to the other, so that the fluxes induced by the I higher frequency current and by the lower frequency current are in the same direction for both inductors.

In the form shown in Figure 10 spaced 1nductors 21 and 22 are fed with current of a relatively high frequency from a source of current 18 corrected for power factor by condenser 28 and an intervening and separate inductor 47 is fed from a source of relatively low frequency current 31 through the secondary of a transformer 32. Here the current from the two frequencies can be applied either separately or concurrently as desired. The same is true of the structure seen in Figure 11 where an inductor 21 fed from a source of higher frequency supply as in Figure 10 and similarly corrected for power factor is surrounded by an inductor 48 which, as in Figure 10, is fed from the secondary of the transformer 32.

It will be evident that all of my forms provide flow of current about the molten metal at different frequencies for the two different but complementary main purposes of heating and stirring and that in the absence of some special reason why these should be successive there is advantage in making them concurrent where the time element is important or where the stirring effect of the heating current used, or the heating effect of the stirring current used is insufiicient to accomplish the purposes required at the time.

My broad invention is effective for heating and stirring to the best advantage and with the greatest economy whether the connections be bridge connections or apply both currents at the same, or approximately the same inductor terminals, or use wholly different inductors. It will be evident moreover that I can accommodate any desired conditions of heating and stirring, using a particularly high frequency if it be desired to heat to a maximum while keeping the metal as quiet as possible and obtaining any proportion of the heating to the stirring by selection of the frequency or by applying the currents concurrently and reducing the strength of one of the currents. This makes possible a very nice adjustment to the needs of treating operations covering the entire range from more holding nearly'free from circulation while maintaining or even vary ing the ten'iperature to treating operations requiring high stirring effect while holding or while varying the temperature.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the Stl11Ct1lI( shown, and I, therefore, claim all such in so far as they fall within the reasonable spirit and scope of my invention.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. The method of melting and stirring molten metal, which consists in circulating two different frequencies of'current about the metal while the metal is molten, to perform heating chiefly by the higher frequency and stirring chiefly by the lower frequency.

2. The method of melting and stirring molten metal, which consists in concurrently circulating two different frequencies of current about the metal while the metal is molten, to perform heating chiefly by the higher frequency and stirring chiefly by the lower frequency.

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4. The method of heating and stirring molten metal in a furnace pool, which consists in circulating electric current of relatively low amperage and high frequency about the quency through the same Winding to perform chiefly the stirring function.

6. The method of heating and stirring molten metal in a furnace pool surrounded by an inductor winding, which consists in circulating electric current of relatively low amperage and high frequency through the winding to perform a heating function largely and in concurrently circulating current of high amperage and of relatively low frequency through he same winding to perform chiefly the stirring funtion, the currents flowing through the same paths and in the same direction.

7. The method of heating and stirring molten metal in a furnace pool, which consists in circulating electric current of relatively low amperage and high frequency about the pool to perform a heating function chiefly and in circulating current of high amperage and of relatively low frequency about the pool to perform a stirring function chiefly, the currents flowing concurrently through the same paths and in part in the same direction.

8. The method of concurrently heating and stirring the molten metal in a furnace pool surrounded by an inductor winding, which consists in concurrently applying current of two different frequencies to the same turns of an inductor and in the same direction and at the same time trapping each of the two frequency supplies through the connection for the other to avoid short-circuiting of the supplies. 9. The method of heating and stirring molten metal in a furnace pool, which consists in inducing currents of relatively high frequencyto flow circumferentially about the pool and concurrently inducing currents of relatively low frequency in the same direction circumferentially about the pool to stir the pool.

10. The method of heating and stirring a furnace pool, which consists in inducing cur-. rents of relatively high frequency to flow circumferentially about the pool and concurrently inducing currents of relatively low frequency in one direction at one level of the pool and in an opposite direction at another pool level circumferentially about the pool to stir the pool.

11. The method of heating and stirring a furnace pool having two connected inductors accessible for contact at one end of each end at the connection, which consists in concurrently applying currents of high frequency and of low frequency to the inductor, one through the two inductors in series and the other through the two inductors in parallel, and protecting against short circuiting of the current applied to the inductors in series.

12. The method of heating and stirring molten metal in.a furnace pool, which consists in inducing current flows of relatively higher and lower frequencies respectively in the pool concurrently throughout the same general paths of flow.

13. The method of heating and stirring a furnace surrounded by an inductor having access at or near its ends and at an intermediate point, which consists in applying highfrequency to the parts of the inductor in parallel connecting at the ends and at the intermediate point respectively, in giving each of the end connections inductance, in balancing the inductances by each other so as to eliminate their inductance to the high frequency current passing through the connections in parallel and in applying lower frequency current to the ends of the furnace coil, whereby the lower frequency current is protected from short circuit through the higher frequency connections by passing through the 1nduetance in series and in compensating the inductor coils for power factor correction suited to the high frequency.

14. The method of concurrently heating and stirring a furnace pool surrounded by an inductor winding, whichconsists in dividing the furnace inductor winding into two inductors located like two adjoining arms of a Wheatstone bridge, with connections corre- Q 'I sponding to the additional two bridge arms, in passing high frequency current through the two inductor arms in parallel and through the other arms in parallel, in applying low frequency curre .t to the inductor arms in series and in providing within the additional arms a trap to reduce or eliminate flow of the low frequency current throughthese arms.

15. The method of concurrently heating and stirring a furnace pool, which consists in dividing the furnace inductor winding into two inductors, in passing high frequency current through these two inductors in parallel, in applying low frequency current to the inductors in series andin providing within the parallel paths of high frequency supply to the ends of the winding inductances mutually offsetting each other for the high frequency and preventing flow of low frequency through the inductances in series.

16. The method of concurrently heating and stirring a furnace pool, which consists in dividing the furnace inductor winding into. two inductors, in passing high frequency current through these two arms in parallel from a common connection to the ends of the wind ing, in applying low frequency current to the inductors in series and in providing within the parallel paths of high frequency supply to the ends of the winding capacity acting as t 'aps to low frequency current flow.

17. The method of heating and stirring a pool of a furnace which consists in applying high and low frequency respectively across alternating opposite terminals of a heatstone bridge arrangement, utilizing two adjoining arms'of the bridge as inductors for a furnace and utilizing the other two arms of the bridge for wave traps against short-circuiting low frequency flow.

18. In an electric furnace a pair of furnace inductors, current supply therefor of two frequencies, one connected through the two inductors in series and the other connected through the two inductors in parallel, and means in the connections of that supply passing through the inductors in parallel for preventing passage of current from the supply for the inductors in series.

19. In an inductor furnace, a pair of connected furnace inductors, power factor corrective condensers for each of the inductors, current supply of two different frequencies for the inductors, the higher frequency connected through the inductors in parallel to their connection to their opposite ends and the lower frequency connected through the inductors in series, and frequency traps in the connections of the currents of higher frequency to the opposite ends of the inductors.

20. In an inductor furnace, a pair of furnace inductors, current supply of two frequencies therefor, connections for applying the current of the higher frequency to the terminals of the inductors, frequency traps in said connections to prevent passage of the current of lower frequency, connection from the other terminal of the supply of higher frequency to the inductors to pass the current from 'said higher frequency supply through the inductors and through the frequency traps in parallel, connections from the supply of lower frequency to the inductor-terminals and condenser power factor correction for the inductors.

21. In an inductor furnace, a pair of furnace inductors, current supplies of two frequencies therefor, connections from one lead of the current supply of higher frequency to the terminals of the inductors, condenser frequency traps in said connections to prevent passage of current from the supply of the lower frequency, connection from the other lead of the supply of higher frequency to the. inductors to pass the current from said higher frequency supply through the inductors and through the frequency traps in parallel, connections from the supply of lower frequency to the inductor terminals and condenser power factor correction for the inductors.

22. In an-inductor furnace, apair of connected furnace inductors having free ends, current supplies of two frequencies therefor, connections from one lead of the supply of higher frequency to the free ends of the inductors, balanced inductance frequency traps in said connections to prevent passage of the current of lower frequency through the connections, connection from the other lead of the current supply of higher frequency to the inductors to pass the current from said higher frequency supply through the inductors and through the frequency traps in parallel, connections from the current source of lower frequency to the free ends of the inductors and condenser power factor correction for the coils.

23. In an induction electric furnace, oppositely wound furnace inductors connected between the inductors having frce ends and adapted to surround a molten change, separate condenser power factor correction for the two inductors, two sources of alternating current supply for the inductors of different frequencies, the source of high frequency supplying the inductors in parallel through the connection between the coils and branch coir nections to the other ends of the inductors, and the source of lower freqnencybeing connected to these other ends of-the inductors and wave traps in the branch connections for protecting against shortcircuiting of the current of lower frequency through the branch connections while permitting free flow of the current of higher frequency through the traps.

24:. In an induction electric furnace oppositely wound furnace inductors connected between the coils and adapted to surround the charge, sepa ate condenser power factor correction for the two inductors, two sources of alternating current supply for the inductors of different frequencies, the higher frequency supplying the inductors in parallel through branched connections and the con nection between the inductors -and the source of lower frequency being connected to the in-.

ductors in series,.and means in the branch connections for protecting against short-circuiting of the current of lower frequency through the branch connections while permitting free flow of the higher frequency through the means.

25. In an electric induction furnace, two inductors adapted to surround the charge, connected together one end ofone coil to one end of the other and their other ends free,

separate condenser power factor correction for the inductors, two sources of current supply for the mductors dlfl ering in frequency,

connections from one source of supply to the junction point of the inductors and branched to the free ends of the inductors respectively, connections from the other source of supply to the free ends of the inductors, tap adjustment for the extent of the inductors covered by the branched connections and frequency traps in the branched connections to prevent short-circuiting of the current from the other source of supply.

26. In an induction electric furnace, an inductor winding, a source of alternating current of one.frequency connected to the mid- (lle of the inductor winding and branched to its ends, respectively, to pass its current through parts of the inductor in parallel, a second source of different frequency connected across the ends of the winding to pass current through the winding in series and opposed inductances in parallel, one in each of the branches of the branched connection, offering substantially no inductance to the currents in the branches but offering the inductance of both to leakage of current from the second source through the branched connection.

27; A coreless furnace inductor winding, a high frequency source of alternating current supply therefor connected to pass current through the winding in series, condenser power factor correction for the winding, condensers in series with the high frequency source of current supply, a low frequency sourceof alternating current supply connected to pass current through the winding in series and an inductance and a capacity in parallel forming a circuit in series with the low frequency source of supply and approximately tuned to the frequency of the high frequency source of supply.

28. A furnace'inductor winding substantially free from inter-threading of transformer iron, high and low frequency source of supply therefor, connections from the two sources of supply to the inductor Windin g and wave traps in the connections, comprising condenser capacity in the high frequency connections and inductance and condenser capacity in parallel substantially tuned to the high frequency and together in series with the connections for the low frequency.

29. A coreless furnace inductor winding,

.condcnser power factor correction therefor,

to the opposite ends of the inductors, inductances in the branched connections to the opposite ends of the inductors compensating each other in parallel use and a higher frequency source of energy connected across said opposite ends of the inductors.

31. A pair of furnace inductors having one end of each connected to the other, and the other ends relatively free, a source of low frequency current connected with the free ends to pass current through the inductors in series, a source of high frequency current connected one lead with the inductors at their common junction, and the other branched to their free ends, means within the connections from the branch for protecting against shortcircuiting of the lower frequency through the branch, switch controlled short circuits for said means and a switch in the low frequency connection;

32. A pair of furnace inductors having one end of each connected to the other, and the other ends relatively free, a source of lowfrequency current connected with the free ends to pass current through the inductors in series, a source of high frequency current connected one lead with the inductors at their common junction and the other lead branched to their free ends, wave traps within the branches for protecting against short-circuiting of the low frequency through the branches, means for short-circuiting the Wave traps and a switch in the low frequency connection.

33. A pair of furnace inductors free from interthreading of transformer iron, high and low frequency current-supplies for said inductors passing both currents through the inductors in parallel, condenser power factor correction for the inductors and wave traps in both circuits to protect against short-circuiting of the high frequency and low frequency, each through the other.

8 1-. A pair of furnace inductors free from interthreading of transformer iron, high and low frequency current supplies for said inductors passing both currents through the inductors in parallel, condenser power factor correction for the inductors and capacity and inductance wave traps respectively in the connections of the two supplies to protect against short-circuiting of the high frequency and low frequency, each through the other.

EDWIN FITCH NORTHRUP. 

